ADM705/ADM706/ADM707/ADM708
Adding Hysteresis to the Power-Fail Comparator
VALID RESET BELOW 1 V VCC
For increased noise immunity, hysteresis can be added to
the power-fail comparator. Because the comparator circuit is
noninverting, hysteresis can be added simply by connecting a
The ADM705/ADM706/ADM707/ADM708 are guaranteed to
provide a valid reset level with VCC as low as 1 V (see the Typical
Performance Characteristics section). As VCC drops below 1 V,
the internal transistor does not have sufficient drive to hold the
PFO
resistor between the
Figure 16.
output and the PFI input, as shown in
RESET
voltage
at 0 V. A pull-down resistor can be connected
externally, as shown in Figure 17, to hold the line low if
required.
7V TO 15V
5V
INPUT
ADP3367
POWER
V
CC
R1
ADM705/ADM706/
ADM707/ADM708
1.25V
–
PFO
TO
+
RESET
PFI
MICROPROCESSOR
NMI
R1
R2
ADM705/ADM706/
ADM707/ADM708
GND
R3
RESET
Figure 17.
Valid Below 1 V
5V
PFO
0V
0V
V
H
V
L
V
IN
Figure 16. Adding Hysteresis to the Power-Fail Comparator
PFO
When
is low, Resistor R3 sinks current from the summing
PFO
junction at the PFI pin. When
is high, Resistor R3 sources
current into the PFI summing junction. This results in differing
trip levels for the comparator. Further noise immunity can be
achieved by connecting a capacitor between PFI and GND. The
equations used to calculate the hysteresis are as follows:
R2 + R3
R2× R3
⎡
⎤
⎛
⎜
⎝
⎞
⎟
⎠
VH =1.25 1+
R1
⎥
⎢
⎣
⎦
VCC −1.25
1.25
R2
⎛
⎝
⎞
⎟
⎠
V =1.25 + R1
−
⎜
L
R3
R1+ R2
R2
⎛
⎝
⎞
⎟
⎠
VMID =1.25
⎜
Rev. G | Page 9 of 12
ADI [ ADI ]
